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RADIO SENSITIVITY — sensitivity of biological objects to the damaging impact of ionizing radiation. Quantitative assessment of radio sensitivity is made by definition of the absorbed doses of ionizing radiation (see) causing a certain effect (see. Doses of ionizing radiation ). So, in many researches it is based on definition of the dose causing death of 50% of the irradiated objects (50% a lethal dose, DL 50 ). R.'s comparison different objects is carried out by means of definition of the absorbed doses of radiation causing in them the identical effect revealed by the same criterion. Studying of mechanisms of the damaging action of ionizing radiation and mechanisms of recovery from the damages caused by it is of great importance in the all-biological plan. At the same time it is necessary for development of methods of antiactinic protection (see) an organism and increase in irradiation efficiency of tumors (see. Radiation therapy ).

The right choice of adequate methods and criteria for evaluation of River is very important. Many reactions to radiation are specific to certain fabrics and systems. E.g., such universal reaction of cells to radiation (see) as the delay of division, easily comes to light in actively proliferating fabrics and cannot be revealed in fabrics where cellular division is expressed poorly or is absent. Therefore for R.'s assessment usually use such accurately registered reactions as survival (or death) cells or organisms.

Range of specific differences of R. of organisms is very wide and is measured by several orders. Not a smaller variety is noted also in R. of different cells and fabrics. Along with radio sensitive structures (the hemopoietic system, intestines and gonads) there are so-called radio steady, or a radior zistentny, fabrics (bone, muscular and nervous).

Speaking about R. of an organism — the main object of studying of medical radiobiology (see) — it is necessary to remember that it varies within one look, depending on age — age R. (most radiochuvstvitelna young and old animals, most radiorezistentna — puberal and newborn), from a floor — sexual R. (as a rule, males more radiochuvstvitelna) and at different individuals of the same population — individual R. Meney are studied seasonal and daily fluctuations of River.

To understand the mechanisms defining natural R. of an organism without what it is impossible to estimate correctly effects of radiation of the person, we will consider consistently cellular and fabric aspects of R. as a cell — the main biological unit, in a cut is implemented influence of the energy absorbed at radiation that in the subsequent leads to development of radiation injury. Among many manifestations of cell activity of the most radio sensitive ability to division is. (Or lethal effect) in radiobiology understand loss as cellular death by a cell of ability to proliferation, and survivors consider the cells which kept ability to unlimited reproduction. Depending on communication of lethal effect with process of division distinguish two main forms of radiation death of a cell: interfazny (coming before cell division or without it) and reproductive (occurring after the first or several subsequent cycles of division). The reproductive form of beam death is characteristic of the majority of cells, including and of cells of many mammals, a cut the structural damages of chromosomes arising in the course of radiation are the main reason. They are found by cytogenetic methods of a research at different stages of a mitosis (more often in an anaphase or in metaphase) in the form of so-called chromosomal reorganizations, or aberrations. Death of such aberrant cells or their descendants happens owing to uneven division or partial loss of vital genetic material because of misconnection of the broken-off chromosomes or a separation of their fragments. Definition of a share of cells with aberation chromosomes is often used as a reliable quantitative index of R. as, on the one hand, the number of such damaged cells accurately depends on an ionizing radiation dose, and with another — this criterion, reflecting its lethal action, well correlates with the quantity of the perishing cells estimated on decline in the ability to a klonoobrazovaniye. By means of these methods on synchronized cellular cultures it was established that R. of a cell changes depending on that what pas of a stage of the life cycle it underwent radiation. For the majority of cells the most radio sensitive stage is the stage of synthesis of DNA (see. Mitosis ). At radiation at the beginning of the presynthetic period survival of cells is considerably bigger. It sharply decreases at radiation at the beginning of a phase of synthesis of DNA and again increases, reaching the largest level at radiation at the end of the period of synthesis.

Interfazny death of a cell occurs to the introduction in a mitosis and for the majority of cells such death is possible only at very high doses. However for some cells, napr, for the low-differentiated hemopoietic elements and lymphocytes, interfazny death happens already at rather low exposure doses. The cells perishing in such way can be revealed in 2 — 6 hours after radiation by means of usual tsitol. methods of a research on various changes (pycnosis of a kernel and fragmentation of chromatin is more often). Calculation of such degenerating cells is also used as a quantitative index of radiation injury.

The molecular substrate responsible for death of a cell, DNA and its complexes with proteins and a nuclear membrane is. The molecular and biological methods of quantitative assessment of damage of DNA used during the studying of mechanisms of radiation defeat of cells and recovery of their viability are developed. The last is caused by the DNA repair which is carried out by special system of enzymes. Efficiency of DNA repair, on modern representations, has the defining value in the cellular River.

Patterns of reactions of cells to radiation remain also at radiation of an organism, R. to-rogo is defined by radio sensitivity of the fabrics, bodies and systems which are directly exposed to radiation. However fabric and furthermore organ and system R. cannot be considered as the sum of cellular effects. Accessory factors exert a great influence on fabric R.: blood supply, volume of the irradiated fabrics, homeostatic control of the regulating systems. All this complicates fabric R.'s assessment, but does not reject basic and leading value of a tsitokineta of the chesky parameters defining type and a measure of beam reactions at all levels biol, the organizations. Cytokinetic characteristic, to be exact proliferative activity of the hemopoietic system, an epithelium of a mucous membrane of intestines and cells of gonads — typical systems of cellular updating — is the reason of their high River. On the contrary, low speed fiziol, regenerations or its absence inherent to a bone, muscular and nervous system, and also a liver causes their relative radiation resistance.

The river of an organism can be characterized on the example of the acute radial illness (see) arising after the general single uniform external irradiation. In this case all systems, bodies and fabrics at the same time are exposed to radiative effects that allows to compare a picture of defeat with an absorbed dose in so-called critical bodies (see). Between an exposure dose, survival and average life expectancy there is a strict dependence determined by distinctions in R. of separate vital systems which defeat is shown in the form of characteristic syndromes — hemopoietic (marrowy), intestinal and cerebral. The hemopoietic syndrome develops at the majority of mammals at general irradiation in doses from 500 to 1000 is glad (5 — 10 Gr). Death comes during from 2 to 4 weeks from an aplasia of the hemopoietic fabric. The intestinal syndrome develops at doses of radiation 1000 — 10 000 is glad (10 — 100 Gr) and comes to an end with a lethal outcome in terms of 4 — 7 days after radiation. At radiation in doses 15 000 I am glad (150 Gr) and more death occurs from a cerebral syndrome in the first days or hours.

At the exposure dose exceeding 100 000 I am glad (1 kgr), animals perish instantly owing to a denaturatsionny inactivation of cells («molecular death»).

Results of impact of ionizing radiation on c. N of page essentially differ from results of radiation of marrow and intestines since mature nervous tissue is not proliferating and consists of the high-differentiated cells which substitution does not happen during all life. The death of nervous cells leading to a cerebral syndrome happens as interfazny defeat or owing to damage of the feeding blood vessels.

The river of marrow is the major factor defining R. of an organism of mammals. Death of its cells at general irradiation even in minimum lethal dose defines death. Characterizing R. of a human body and mammals, usually mean the doses causing their death at the phenomena of the hemopoietic (marrowy) syndrome.

As for the fabrics and bodies carried to radio resistant, it is impossible to tell that they do not react to impact of ionizing radiation at all. And in cells of not proliferating fabrics under the influence of radiation there are damages of chromosomes which are not realized, however, due to the lack of cellular division, and therefore not affecting viability of the based cells. But if artificially to cause proliferation of such cells (e.g., cells of a liver by its partial resection), then it turns out that they as if «remembered» radiative effects, and in the sharing elements of the regenerating body aberation chromosomes, and an exit of such damaged cells to unit of a dose easily are found it is same, as well as in cells of radio sensitive bodies. There are bases to believe that «preservation» of radiation defeat of chromosomes in cells of the bodies and systems which are characterized by low level fiziol of proliferation, is the reason of their functional inferiority in the remote terms after radiation; it promotes finally reduction of life expectancy of an organism and development of malignant tumors.

Artificial change of R., or management P., is carried out as towards its decrease (antiactinic protection), and towards increase (radio sensitization). The physical and chemical agents used for antiactinic protection received the name of radio of protectors (see), and applied to a sensitization to radiation — radio sensitizers (see. the Radio sensibilizing substances ).

An important control facility radio sensitivity, including for the purpose of the directed R.'s modification normal and tumoral fabrics at radiation therapy (see), the regulation of fabric tension of oxygen based that R. of fabrics increases at its increase is and decreases at its decrease (see. Oxygen effect ).

Bibliography: V.'s bond, Flidner T. and Arshambo. Radiation death of mammals, the lane with English, M., 1971; Comparative cellular and specific radio-sensitivity, under the editorship of Century. The bond and T. Sugakhara, the lane with English, M., 1974; E y - d at with L. X. Nonspecific reaction of cells and radio sensitivity, M., 1977; Yarmonenko S. P. Radiobiology of the person and animals, M., 1977.

S. P. Yarmonenko.